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Full text of "Handbook Of Chemical Engineering - I"

76
CHEMICAL ENGINEERING
wall or floor attachment. Kimball andBarr:
The following percentage efficiencies are given by
Common bearing (singly)...... 96-98   Worm gear: varies with thread angle
Do., long lines of shafting....
Roller bearings...............
Ballbearings................
Spur gear, including bearings:
Cast teeth................
Cut teeth.................
95 98 99
92 95
Cast teeth.....................        92
Cut teeth......................        95
Belting..........................   96-98
Pin-connected chains (bicycle)......   95-97
High-grade transmission chains.....   97—99
Rope Drives.—Transmission ropes are usually cotton, hemp or manila, three to six strand, according to size. Cotton is most flexible but weakest. Pulley diameters should be at least 36 times the rope diameter. Rope diameters
above 1M in. should be avoided.    The rope may be continuous or a series of ropes may run side by side.   The latter system is more assured against total breakdown, is flexible and cheaper to maintain: but it requires many splices and is not adapted for vertical transmission, and provision should be made for   equalizing   stresses.    Efficiencies vary from 0.55 to 0.95 according to the complexity of the drive.   Ropes  are cheaper than belts but belt pulleys cost less than rope sheaves.    Wire   ropes are  made up   of   strands,   reversely twisted to form the rope.   Standard rope has a hemp core.   Large sheaves must be used, especially for high speeds. Hydraulic  power transmission  is used for  slow-moving, powerful machinery requiring accurate control, or where load factors are very low.   Efficiencies of hydraulic  operation   range up to 0.85, the chief loss being that due to the friction at   cup   leathers used for   plunger   packings.   For  direct   steam  pumps   having  the   same number of fluid cylinders as steam cylinders, the ratio of areas of  steam and fluid cylinders is j^ - —;, where pw and p8 are fluid and steam pressures, pounds  per square inch, and e = mechanical efficiency of pump, say  0.75. For   double-acting   cylinders, the volume of fluid discharged per minute is V = AwSe, where 8 is piston speed, inches per minute, and ev is the volumetric-efficiency, 0.90 or more for new pumps.   The volume is half this for single-acting cylinders.   The power required to drive the fluid cylinder is Vwpw ^ 14,300* hp. where w = weight of 1 cu. in. of fluid, pounds.   Piping should be designed for a fluid velocity of 3 to 5 ft. per second, or a pressure drop of 10 Ib per square inch per mile.   A velocity of 100 ft. may be allowed through control valves.   Non-freezing fluids must often be used.   Double extra-strong wrought
_w«-    —    --°
2      o:      ro
Feet per Minute/ FIG. 36.—Power transmitted by cotton rope